The combustion/electrical hybrid or electrical only vehicles notably include high power batteries. Such batteries are used to drive an electric motor with alternating current via an inverter. The voltage levels needed for such motors reach several hundreds of Volts, typically of the order of 400 Volts. Such batteries also have a high capacity in order to favor the range of the vehicle in electric mode.
To obtain high powers and capacities, several groups of accumulators are placed in series. The number of stages (number of groups of accumulators) and the number of accumulators in parallel in each stage vary as a function of the voltage, of the current and of the capacity desired for the battery. The association of a plurality of accumulators is called an accumulator battery. The electrochemical accumulators used for such vehicles are generally of lithium ion type for their capacity to store a considerable amount of energy with a weight and a volume that are contained. The lithium-ion iron phosphate, LiFePO4, battery technologies are the object of significant developments because of an intrinsically high level of safety, to the detriment of a slightly lower energy storage density. An electrochemical accumulator usually has a nominal voltage of the following order of magnitude:
3.3 V for a lithium-ion iron phosphate, LiFePO4, technology,
4.2 V for a technology of lithium-ion type based on cobalt oxide. The invention can also be applied to supercapacitors.
FIG. 1 represents a lithium-ion accumulator battery Bat known from the prior art. The battery Bat is made up of four stages Et1, Et2, Et3 and Et4 connected in series. Each stage comprises four similar accumulators, connected in parallel. The terminals of the accumulators of one and the same stage are connected together via large section electrical connections. Each stage is also connected to the adjacent stages via large section electrical connections in order to allow high currents to pass, corresponding to the sum of the currents from the accumulators of a stage. One or more loads are intended to be connected to the N and P terminals of the battery 1.
The voltage at the terminals of the four stages is respectively denoted U1, U2, U3 and U4. In this arrangement, the total voltage U between the N and P terminals of the battery 1 is the sum of the voltages U1, U2, U3 and U4. The current passing through each accumulator of the fourth stage Et4 is respectively denoted I1, I2, I3 and I4. The current I generated on the P terminal of the battery Bat is the sum of the currents I1, I2, I3 and I4. A charge equalizing circuit Eq is connected to the terminals of each stage of the battery Bat.
Throughout the life of the battery, certain faults may appear on some accumulators that make up the battery. A fault on one accumulator is generally reflected in the short-circuiting of the accumulator, or an open-circuiting, or in a significant leakage current in the accumulator. It is important to know the impact of the failure of an accumulator on the battery. An open-circuit or short-circuit can cause an overall failure of the entire battery.
In the case of the appearance of a significant leakage current in an accumulator of a stage, the battery behaves like a resistor which provokes a discharging of the accumulators of the stage considered to zero. The risks of a fire starting are low because the energy is dissipated relatively slowly. In lithium-ion technology, the discharging of the accumulators of the stage to a zero voltage damages them which means replacing them in addition to the accumulator that initially failed. When an accumulator forms a short circuit, the other three accumulators of the stage will initially discharge into this accumulator, because of the large section of the electrical connections between them. The fuse placed in series with the short-circuited accumulator will stop the spurious discharging of the other three accumulators.
In order to protect the battery Bat from the consequences of a short circuit in an accumulator, each accumulator has a fuse which is connected to it in series. When an accumulator forms a short circuit, the current passing through it increases significantly and causes its series fuse to blow in order to protect the rest of the battery Bat. In the absence of a fuse, the energy dissipation in the short-circuited accumulator would cause it to overheat together with the other accumulators being discharged. Such a dissipation could be the cause of a fire being started. The lithium-ion technologies are particularly at risk when a stage comprises a large number of accumulators in parallel to store significant energy. Cobalt oxide is known as a strongly reactive chemical. Iron phosphate is known to be the safest chemical. The use of fuses is therefore particularly appropriate for these technologies, particularly for iron phosphate which tolerates a certain overvoltage.
However, the presence of fuses in series between the accumulator stages induces not-inconsiderable losses, particularly challenging for embedded applications.
The document WO2011/003924 describes a battery structure that makes it possible to eliminate the losses induced by a protection system in the normal operation of the battery, and that also makes it possible to ensure a continuity of service of the battery when an element of the battery is short-circuited or open-circuited.